<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div
        {
            background-color: green;
            width: 700px;
            height: 700px;
            border: 2px black solid;
            box-sizing: border-box;
            display: flex;/*弹性布局模式*/
            /* 水平方向 */
            /* justify-content: end; */
            justify-content:center;
            /* justify-content: start; */  /*默认*/
            /* justify-content:space-between; */
            /* 竖直方向 */
            /* align-items: start; */
            align-items: center;
            /* align-items: end; */

        }
        div span
        {
            background-color: red;
            width: 100px;
            height: 100px;
        }
    </style>
</head>
<body>
    <h3>弹性布局</h3>
    <div>
        <span>1</span>
        <span>2</span>
        <span>3</span>
    </div>
</body>
</html>